Liquid crystal compositions are used in various electro-optical devices which involve the modulation of electromagnetic radiation, such as light valves and transmissive or reflective optical display devices. Such light valves are controlled by an electric field and operate when the nematic liquid crystal material is in its mesomorphic state.
Mesomorphism has been described as a state of matter with molecular order between that of a crystalline solid and a normal liquid. Crystalline solids are characterized by a non-random distribution of the molecules and a three-dimensional order in the location of the individual molecules within the crystal lattice. Normal liquids generally show isotropic behavior, for example, to light, due to the fact that the molecules of the liquid are randomly oriented.
In the mesomorphic state or mesophase of liquid crystal compositions, which are comprised of rod-shaped molecules, the directional arrangement of at least a part of the molecules is non-random. Among the various types of liquid crystal compositions, nematic liquid crystals are characterized by the fact that the long axes of the molecules maintain a parallel or nearly parallel arrangement to each other such that a one-dimensional order exists. Nematic liquid crystal compositions are usually characterized by a turbid appearance.
The mesophases of liquid crystal compositions exist over a temperature range which is dependent on the specific nature of the composition and molecular structure. Below this range the compositions become crystalline solids and above this range, the preferred directional alignment of the molecules is destroyed and a normal liquid having isotropic behavior results. Both of these phase changes are characterized by sharp transition points.
In the mesomorphic state, the anisotropic properties of the individual molecules are conferred upon the bulk material. In regard to dielectric properties, the dielectric constant (.epsilon..sub..parallel. ) parallel to the long axis of the molecules can be larger or smaller than the dielectric constant (.epsilon..sub..vertline. ) perpendicular to the long axis of the molecules. If .epsilon..sub..parallel. is greater than .epsilon..sub..vertline. , such that .epsilon..sub..parallel. - .epsilon..sub..vertline. &gt; 0 or .epsilon..sub..parallel./.epsilon..sub..vertline. &gt; 1, then the composition in question is said to have a positive dielectric anisotropy. On the other hand, if .epsilon..sub..parallel. is less than .epsilon..sub..vertline. , such that .epsilon..sub..parallel. - .epsilon..sub..vertline. &lt; 0 or .epsilon..sub..parallel./.epsilon..sub..vertline. &lt; 1, then the composition is said to have a negative dielectric anisotropy.
This dielectric anisotropy is responsible in part for the utility of liquid crystalline compounds in various electrooptical devices which involve the modulation of light, such as light valves and optical display devices. Such light valves typically are controlled by an electric field and operate when the liquid crystalline material is in its mesomorphic state.
The anisotropic molecules can be aligned perpendicularly or uniaxially parallel to a surface giving a transparent appearance, and when an external magnetic of electric field above a threshold value is applied perpendicular to the surface, molecules with a negative dielectric anisotropy tend to orient perpendicularly to this field. However, this orientation is impeded by the presence of ions moving in the field which cause constant movement of the liquid crystal molecules (these molecules behaving as groups about 10.sup.-.sup.5 cm. in size) which is a dynamic state resulting in the scattering of light. Thus, the application of an electric or magnetic field brings about a change from a relatively transparent optical state to a translucent dynamic scattering state.
From Helfrich's theory [J. Chem. Phys., 51, 4092 (1969)] of the threshold voltage for the onset of electrohydrodynamic instabilities in liquid crystals the following criteria are extracted: ##EQU1## for the electric field applied perpendicular to the long axis of the liquid crystalline molecules and ##EQU2## for the electric field applied parallel to the long axis of the molecules. In expressions I and II above, .sigma. refers to the conductivity and the subscripts .parallel. and .perp. refer to the component of the anisotropic material parameter relative to the direction of axis of preferred molecular interaction as used previously in connection with the dielectric constant. In order to create electrodynamic instabilities in a liquid crystalline material so as to result in intensive turbulence with concomitant high light scattering (dynamic scattering), the anisotropies of .sigma. and .epsilon. have to fulfill the above relations in which C accounts for the viscosity anisotropy.
Liquid crystalline materials with a negative dielectric anisotropy (.epsilon..sub..parallel./.epsilon..sub..vertline. &lt; 1) will be oriented with the long axis perpendicular to the applied field and relation I above applies. On the other hand, materials with a positive dielectric anisotropy (.epsilon..sub..parallel./.epsilon..sub..vertline. &lt; 1) will be oriented with the long axis parallel to the applied field and relation II applies. However, relation II requires that .sigma..sub..parallel./.sigma..sub..vertline. &gt; 1 which is rarely ever observed in liquid crystalline materials. Thus, materials having a positive dielectric anisotropy are generally unsuited for dynamic scattering applications.
Of all liquid crystalline materials having a negative dielectric anisotropy, some are better suited than others for use in dynamic scattering applications and some are not useful at all because, for example, the magnitude of .DELTA..epsilon. relative to .DELTA..sigma. is not appropriate. Accordingly, there is a need in the art for a means of rendering nematic liquid crystalline materials having a negative dielectric anisotropy suitable for dynamic scattering.